A. Field of the Invention
This invention relates to improved conductivity sensors of the type employed in recirculating water conditioning systems, such as disclosed in U.S. Pat. No. 3,361,150 issued Jan. 2, 1968 to J. E. Horner where fresh feed water is added to the system to limit the concentration of dissolved solids.
B. Prior Art
In most systems, the conductivity of a solution is monitored using either of two basic methods. One measures conductivity directly by maintaining a fixed voltage between electrodes immersed in solution so that the resulting current flow is directly proportional to the conductivity. Conversely, the electrodes may be supplied with a constant current flow so that the potential between them is directly proportional to the resistivity of the solution, which is the reciprocal of its conductivity.
In many applications, a simple two electrode system has sufficed. As pointed out in the aforementioned Horner patent, the application of an alternating current excitation is preferred for continuous monitoring to prevent the corrosion and buildup of electrolysis products at the electrode surface interface with the solution. Nevertheless, in many process streams, such as those employing cooling water evaporation for air-conditioning systems for commercial and industrial buildings, solid impurities carried by the stream are deposited upon the electrode surfaces. Over extended periods of use, the fouling of the electrodes by the buildup of these impurities can introduce a substantial impedance across the interface between the electrode metal and the solution, thus spoiling the accuracy of the conductivity reading by indicating a much lower value than is actually present in the solution itself. Since such a result could have serious consequences for the entire recirculation system, operation had to be periodically interrupted to permit inspection and cleaning of these electrodes to insure proper response.
More recently, various four electrode conductivity monitoring systems have been developed to alleviate these problems. In these arrangements, two electrodes are coupled through a high impedance circuit to determine a potential relationship, whereas a relatively low impedance constant current source supplies the other two. With the impedance in the voltage circuit much higher than the interface resistance between the electrode and the solution due to fouling or corrosion, the existing potential is primarily due to the voltage drop produced by the current flow through the solution between the other two electrodes. By holding the current constant between those electrodes, the measured voltage between the other two is then directly proportional to the resistivity of the solution, even with considerable fouling by impurities and corrosion products. However, in order to produce an output signal proportional to conductivity, an electronic reciprocal generator with its inherent cost and inaccuracy is needed to convert the resistivity signal.
Also, as explained in the aforementioned patent, the voltage between most metal electrodes should not exceed their free corroding potential, typically about 40 millivolts, so as to not foreshorten their usable life. On the other hand maximum signal strength decreases the noise effects of stray currents and the like to make the measurements more precise. Therefore, the voltage produced by a constant current might exceed permissable corrosion potential limits when the electrodes become fouled unless the initial operating levels are set well below permissable limits, thus decreasing the available signal-to-noise ratio.